As a business we were forced some years ago to legalize all our computers buying a volume license for our computers, which costed like US$ 220 per license. The advantage of this license was that it is eternal, and you can transfer it to a new computer if your renew your computers.
The OEM which comes with the computers of Ripley and Saga, is legal, but only allows to use on that particular computer.( Also for business use ) Once you replace it, you loose the license.
But we found out that these computers for daily office-life, are more than sufficient, and taking into account that a VLSC ( volume license ) costs US$ 220, it works out cheaper to but computers with Windows preinstalled, which cost (including the OEM windows license ) around S/. 1000 - 2000 soles for a decent one.
Office legal, cost around US$ 250 per license ( for business ) and S/. 120 on mercadolibre Office365 per year, for home use.
It highly depends on what you use your computer for. Homeuse, or business use.